Calculate phase agreement over symmetry-allowed origin shifts

[tjlane]

! This PR was vibe-coded.

Stack

Part 5 of 11 for #31. Depends on #354. Merge bottom-up.

Next branch: codex/phase-alignment-06-bounded-fft.

What this implements

Adds the exact numerical foundation for a symmetry-constrained origin search: an integer basis for the continuous polar lattice, per-coset Fourier terms, Nyquist-complete intrinsic-dimensional grids, a materialized inverse FFT, and a memory safety estimate.

The implementation supports zero through three floating dimensions and oblique axes such as R 3:R.

Tests

Tests compare FFT grid values directly against the circular-correlation objective in one, two, and three dimensions; cover exact integer bases, zero-weight frequencies, oblique rhombohedral axes, and the materialized-memory guard.

Reviewer focus

• The integer polar-basis construction and its behavior for R settings.
• Fourier sign, frequency wrapping, inverse-FFT scaling, and grid-size choice.
• The distinction between discrete origin cosets and continuous polar coordinates.

Related: #31 and #174.